throw.xml Add an example

[mmalferov]

Can we add an example that shows how the Fiber::throw() method passes an exception to the fiber?

[Girgias]

@trowski would you be able to write such an example?

[trowski]

The example supplied looks fine. This was a PR, not an issue. 😄

[lacatoire]

LGTM

[jordikroon]

I have changed this to an informalpara as there is this is the default example. Plus removed the outer <para>.